At present, a chemical mechanical polishing (CMP) is an indispensable technology for flattening a wafer to achieve high performance and high integration of a semiconductor device.
In the CMP process, a wafer is placed on a pad stuck to a polishing bed so that a surface being polished of a wafer may come into contact with the pad, and, with a pressure head pressed against the wafer to apply a constant weight thereon and with a polishing composition supplying on a pad surface, the pad and the pressure head are rotated to polish the wafer.
The polishing composition is an aqueous slurry in which a polishing agent is dispersed, and, depending on a material of a film formed on a surface being polished of the wafer, an appropriate one is selected from various polishing agents. Among them, a polishing agent made of silica such as colloidal silica and fumed silica is generally used (Japanese Unexamined Patent Publication JP-A 52-47369).
Among the silica base polishing agents, the colloidal silica is excellent in the dispersion stability in water. Accordingly, a polishing composition that is an aqueous dispersion solution of colloidal silica, as far as a concentration of the colloidal silica is in an appropriate range, even after a long storage, is difficult to cause agglomeration of the colloidal silica. However, since the colloidal silica is relatively slow in the polishing speed, there is a problem in that it takes a long time to polish a wafer.
Accordingly, in combination with the colloidal silica, a polishing accelerator such as an organic acid, an oxidizing agent such as hydrogen peroxide, an corrosion inhibitor such as a benzotriazole compound and a surfactant are used. Furthermore, since the colloidal silica is industrially produced from sodium silicate as a raw material and resultantly contains sodium as an impurity, a wafer is likely to be contaminated at the time of polishing.
Accordingly, it becomes necessary to refine colloidal silica to make higher in the purity. Thus, since when the colloidal silica is industrially produced, a refining process to make higher in the purity becomes necessary, the productivity is deteriorated and the producing cost goes up.
On the other hand, fumed silica is faster in the polishing speed than the colloidal silica. In addition, since the fumed silica is synthesized by burning silicon tetrachloride in an oxygen hydrogen flame, the impurity is less contained and the cost is industrially less expensive. However, the fumed silica is insufficient in the aqueous dispersibility.
Accordingly, in a polishing composition that is an aqueous dispersion solution of fumed silica, owing to external loads such as piping load (collision with an inner wall of piping) when supplying to the CMP process, load (pressure owing to a feed pump) of a feed pump, load (pressure load owing to a pressure head) of the pressure head or an environmental condition when conveying, the fumed silica is agglomerated. Furthermore, during the long-term storage, the fumed silica tends to be agglomerated.
The fumed silica enlarged by the agglomeration causes a lot of polishing flaws on the wafer. Such polishing flaws disturb the reliability of the electrical connection of the wafer. Particularly when the polishing flaw that is 0.2 μm or more in diameter is caused more than 100 on one wafer, the wafer becomes a defective product to deteriorate the yield of the polishing step.
In view of high polishing speed of the fumed silica and an advantage in the cost thereof, various technologies have been proposed to improve the aqueous dispersibility of the fumed silica.
For instance, there is a method where water and fumed silica are mixed under application of a high shearing force to obtain an aqueous dispersion solution containing the fumed silica at a high concentration, water is added to the aqueous dispersion solution to dilute, and thereby a polishing composition containing a desired concentration of fumed silica is obtained (refer to, for instance, Japanese Examined Patent Publication JP-B2 2935125).
Furthermore, there is a method where, under application of a high shearing force, acid and fumed silica are sequentially added to water and mixed, water is further added thereto, and an alkali aqueous solution is added thereto to obtain a polishing composition containing fumed silica (refer to, for instance, Japanese Examined Patent Publication JP-B2 2949633).
Still furthermore, there is a method where, to water of pH 2 to 4, under application of a high shearing force, fumed silica is added so that a concentration may be 40 to 60% by weight, water is further added to adjust the viscosity to 2 to 10000 cps, followed by stirring for 5 min or more under application of a low shearing force, further followed by adding water to adjust a concentration of fumed silica to 10 to 38% by weight, still further followed by adding alkali under strong stirring to adjust the pH to 9 to 12, and thereby a polishing composition containing fumed silica is obtained (refer to, for instance, Japanese Unexamined Patent Publication JP-A 2001-26771).
However, the polishing compositions described in JP-B2 2935125, JP-B2 2949633 and JP-A 2001-26771, though improved in the aqueous dispersibility of fumed silica in comparison with existing one, are not yet in a sufficiently satisfying level. Accordingly, under the external load and/or during long storage, the fumed silica is unavoidably agglomerated.